Sol-gel has been employed as a monolithic gel deposition on a variety of substrates. See, for example, U.S. Pat. No. 4,652,467 and U.S. Pat. No. 5,224,972, both issued to Brinker et al. In this process, metal alkoxides of network forming cations, e.g., Si, Al, B, Ti, P, and optionally soluble salts of modifying cations, are used as glass precursors. In alcoholic solutions catalyzed by additions of acid or base, the alkoxides are partially or completely hydrolyzed and then polymerized to form molecules of glass-like oxide networks linked by bridging oxygen atoms. This technique is readily adapted to preparation of multicomponent oxide solutions as well as single component systems.
The net reactions which describe this process are generally represented as: ##STR1##
where x in reaction 1 can be varied, e.g., from about 1-20. Generally, reaction 2 does not go to completion, i.e., colloidal particles of anhydrous oxides do not result. When the growing polymers link together to form an infinite network, the solution stiffens to a gel.
The chemistry involved in the formation of these monolithic gels is well documented in the prior art. See, e.g., (1) Brinker et al, "Sol-gel Transition in Simple Silicates", J. Non-Cryst. Solids, 48 (1982) 47-64; (2) Brinker et al, "Sol-gcl Transition in Simple Silicates II", J. Non-Cryst. Solids, 63 (1984) 45-59; (3) Schaefer et al, "Characterization of Polymers and Gels by Intcnediatc Anglc X-ray Scattering", presented at the International Union of Purc and Applied Chemists MAC-RO'82, Amherst, Mass., Jul. 12, 1982; (4) Pettit et al, Sol-Gcl Protective Coatings for Black Chrome Solar Selective Films, SPIE Vol. 324, Optical Coatings for Energy Efficiency and Solar Applications, (pub. by the Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash.) (1982) 176-183; (5) Brinker et al, "Relationships Between the Sol to Gel and Gel to Glass Conversions", Proceedings of the International Conference on Ultrastructure Processing of Ceramics, Glasses, and Composites, (John Wilcy and Sons, N.Y.) (1984); (6) Brinker et al, "Conversion of Monolithic Gels to Glasses in a Multicomponent Silicate Glass System", J. Materials Sci., 16 (1981) 1980-1988; (7) Brinker et al, "A Comparison Between the Densification Kinetics of Colloidal and Polymeric Silica Gels", Mat. Res. Soc. Symp. Proc. Vol. 32 (1984), 25-32; all of which disclosures are entirely incorporated by reference wherein. For example, much work has been done in characterizing the relationship between the properties of a monolithic, bulk gel prepared by these systems and of the properties of the solution from which such gel is made. For instance, the relationship between solution characteristics such as pH and water content for a given solution chemical composition and the size and nature of the polymer which results in solution, and the relationship between such polymer properties and the characteristics of the finally produced gel, e.g., the degree of crosslinking, the porosity of the gel, etc., have been well studied and discussed in these references.
The fact that gel formation can be retarded by making the solutions sufficiently dilute, e.g., less than 10% equivalent oxides, is known. In such dilutions, more typically 2-5% equivalent oxides, the solution can be applied to various substrates by conventional processes. Under such circumstances, the partially hydrolyzed glass-like polymers react chemically with the substrate surface, thereby achieving complete wetting.
The physical properties of sol-gel materials are tailored through stoichiometry, aging, drying conditions and method of deposition. Emphasis for examining these parameters has been on silicate-based systems, which has led to microporous monoliths and thin films (pore size &lt;2 nm). The most prominent applications of sol-gel synthesis have been the development of mesoporous (pore size from 2 nm to 50 nm) materials that possess well-defined pore morphology. To generate this pore morphology, a method known as surfactant templating has been devised. This approach is based on the ability for a ternary system, consisting of water, ethanol and surfactant, to develop a three dimensional structure (or a lyotropic phase) that may be described as cubic, hexagonal, lamellar or isotropic, depending upon the molar ratio of the three components. The formation of these phases is sometimes referred as liquid crystal templating. In general, the introduction of a hyrdolyzed silicon alkoxide precursor, once hyrolyzed, infiltrates the water rich regions and forms in inorganic `shell` around the hydrophobic surfactant. Upon drying and heating in excess of 400.degree. C., the organic surfactant phase is removed, leaving behind the inorganic, silica shell with porosity defined by the once present surfactanit phase. The pore sizes range from 2 nm to 100 nm depending upon the nature of the surfactant. The silica wall thickness ranges from 1 nm to 10 nm, which relies on processing parameters such as aging, pH and temperature.
However, none of the known uses of sol-gel chemistry in thin film deposition contemplates the use of sol-gel as a permeation layer for an electrical micro-array devices. Current permeation layers for electric micro-arrays are organic monomers or polymers with undefined pure size and porosity that swell when exposed to an aqueous solution. The previously not contemplated use of sol-gel as a permeation layer for an electrical micro-assay device solves the above limitations of organic permeation layers by providing a permeation layer that has controllable porosity and pore size and is not susceptible to swelling when exposed to an aqueous solution.